What is meant by heuristic optimisation?  Discuss the main heuristics that are applied during query optimisation. 

Ans:  In heuristic optimization, heuristics are employed to reduce the cost of optimization in place of analyzing the number of different plans to make out the optimal plan. For instance, a heuristic optimizer would make use of the rule 'Perform selection operation as early as possible' without finding out whether the cost is reduced by this transformation. Heuristics approach generally assists to reduce the cost but not all the time. The major heuristics that are applied throughout query optimization are:

• Pushes the selection and projection operations down the query tree
• Left-deep join trees - convenient for pipelined evaluation
• Non-left-deep join trees